This article is from the Misc Bicycles FAQs, by various authors.
Thirst is similarly suppressed during strenuous exercise. Though
trained athletes are generally able to judge how much to drink, large
fluctuations in need can occur due to fatigue, temperature, altitude,
humidity, air flow, fitness, food and the type of fluid consumed.
Dehydration be deceptive because it feels exactly like fatigue. Late
in a hard ride, when you feel tired, it may simply a case of
dehydration. Don't sell yourself short, drink up before getting
A fluid loss of as little as three percent of total body weight
(4.5 of 150 lbs.) can measurably effect performance. Dehydration
impairs thermoregulation through reduced sweating and blood
circulation. As dehydrated blood gets thicker it creates cardiac
stress which impairs the metabolic processes necessary for physical
Losing more than a few pounds on a ride, or more than a couple of
pounds overnight, is a good indicator that you didn't drink enough.
With experience it will become easier to keep within a few pounds of
pre-race weight even on long rides in hot weather.